The invention relates broadly to the treatment of water by aeration. The aeration treatment of water has been used to treat waste water within lagoons and to upgrade the quality of natural bodies of water, such as lakes.
In the majority of prior design criteria evaluations for aerated lagoons, focus has been placed almost entirely on the satisfaction of waste biochemical oxygen demand (BOD). That is, the amount of supplemental aeration or oxygen which was to be supplied to a lagoon was calculated by the amount which the waste strength exceeded the oxygen available from photosyntheses and atmospheric aeration. The mixing of the waste water to maintain solids in suspension while aerating the body of water was ignored until recently. In an article entitled "How to Design Aerated Lagoon Systems to Meet 1977 Effluent Standards-Soluble Substrate Removal Relationships" by Sam C. White and Linvil G. Rich at pages 82-83 of Water and Sewage Works, April 1976, mixing was included as a design parameter for an aerated lagoon.
Waste water treatment aeration systems generally utilize either diffused air aerators or mechanical aerators. A diffused air type aerator introduces air or pure oxygen into water via submerged forced diffusers or nozzles. Mechanical type aerators generally agitate the water so as to promote solution of air from the atmosphere into the water. These conventional aerators are designed primarily from the standpoint of introducing a certain amount of oxygen into the water being treated. The mixing of the water and the introduced oxygen has not been a design criteria and, hence, conventional prior aerator systems have inefficient mixing capabilities. For example, in a one acre aerated pond for treating domestic waste water, a pond which has a depth of ten feet and a 3:1 side wall slope and a volume of 432,000 cubic feet, conventional aerator sizing procedures would require approximately 200-400 horsepower of surface area capacity or over 1,000 horsepower for a diffused air system. Such high power systems result in the oxygen requirement of the pond being satisfied fourfold, while probably not causing a complete mixing of the pond so as to assure scouring velocities of 0.5 feet per second throughout the pond.
Mechanical surface aerators also exhibit an additional problem in that they have separate and generally conflicting mixing vectors. That is, the force vectors from the aerators cannot generally be managed or manipulated and, hence, with closely spaced aerators, mixing vectors tend to cancel out.
Another problem with most conventional aerator systems is that their oxygen transfer is effectively limited by their inability to properly mix the waste. Most conventional aerator systems tend to have a very limited core of influence, that is, the conventional aerators cause a high dissolved oxygen concentration close to the units themselves because of the inability of the units to mix. Thus, such conventional systems tend to create a condition of oxygen oversaturation instead of under saturation, which would promote oxygen transfer. Many conventional aerators are tested in small tanks and the test measured oxygen transfer rate at zero DO is then reported as a standard for comparision. This standard, however, is not accurate in field applications since in field applications, the dissolved oxygen is required in a much larger volume. In field installations, the conventional units tend to overaerate the nearby region and, thereby, to effectively work against themselves. The dissolved oxygen is not effectively reaching outlying regions.
Other problems also arise due to the failure of the prior art aeration systems to adequately mix the waste water being treated. Without adequate mixing, for example, when 0.5 fps horizontal velocity is not attained, aerated lagoons may develop adverse conditions that effect performance such as hydraulic short circuiting and/or sludge solids build-up. As waste water enters an unmixed lagoon, a certain fraction of waste may move directly to the outlet without adequate time of interaction with other lagoon contents for adequate treatment. If the pond is completely stagnate, influent velocity effects or momentum may set up such a "short circuit" current and this problem may be exasperated by thermal stratification effects. Another problem is a sludge blanket build-up. Under mixed lagoons, cells may develop aerobic-anaerobic regions analogous to a facultative pond. Anaerobic sludge deposits found in undermixed lagoons may produce noxious odors such as H.sub.2 S and/or NH.sub.3 gases.
These gases may carry a high concentration of oxidizable organic material (i.e., BOD) into the water. Finally, the release of these gases together with the CH.sub.4 and N.sub.2 may contribute to floating solids and a tendency to "belch" these into the lagoon or cell outlet. This mass of material may be transported into subsequent treatment cells or out as a final effluent.
Sludge deposits deposited in a lagoon must be regarded as lost from control. Anaerobic digestion may occur when temperatures permit, with the result that performance may become seasonal. The system is effectively out of operational control, when effluent suspended solids in BOD concentrations become a function of such factors. Such a system cannot reliably assure the degree of performance needed to meet the ordered quality standards of today.
The method in accordance with the present invention overcomes the above deficiencies of conventional systems by both aerating a body of water and causing mixing of the aerated water at efficient power levels.